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Lecture16

# Lecture16 - Physics 132 Introductory Physics Electricity...

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1 February 13, 2008 Physics 132-Winter 2008 Prof. Jim Beatty-Ohio State 1 Physics 132 Introductory Physics: Electricity and Magnetism Winter Quarter 2008 Lecture 16 February 13, 2008 Physics 132-Winter 2008 Prof. Jim Beatty-Ohio State 2 Capacitors, Resistors, and Power Key results so far: –Capacitors: Q=CV •Parallel: C eq = C 1 +C 2 •Series: 1/C eq = (1/C 1 + 1/C 2 ) –Resistors: V=IR •Power Dissipation: P = I 2 R =V 2 /R Let’s apply these to some circuits…

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2 February 13, 2008 Physics 132-Winter 2008 Prof. Jim Beatty-Ohio State 3 Single Loop DC Circuit P E + - I R V has a value everywhere, once you pick where V=0. A circuit consists of one or more closed loops. Since V is independent of the path, Δ V is 0 around any closed loop. To write an equation for the circuit, start at P and walk around the circuit back to P. At an EMF device: Add + E going from - to + Add - E going from + to - At a Resistor: Add -IR going in direction of I Add +IR going opposite from I February 13, 2008 Physics 132-Winter 2008 Prof. Jim Beatty-Ohio State 4 Single Loop DC Circuit P E + - I R At an EMF device: Add + E going from - to + Add - E going from + to - At a Resistor: Add -IR going in direction of I Add +IR going opposite from I Clockwise: Going from P across battery: +E Going through resistor along I: -IR Back to P: Sum of all voltage changes is zero. E - IR = 0
3 February 13, 2008 Physics 132-Winter 2008 Prof. Jim Beatty-Ohio State 5 Single Loop DC Circuit P E + - I R At an EMF device: Add + E going from - to + Add - E going from + to - At a Resistor: Add -IR going in direction of I Add +IR going opposite from I Counter-clockwise (changing direction of I to match): Going from P through resistor along I: -IR Going across battery from + to -: - E Back to P: Sum of all voltage changes is zero. -E - IR = 0 - sign comes from our assumed current direction. When we solve for I, it will be negative, indicating direction is opposite our choice. February 13, 2008 Physics 132-Winter 2008 Prof. Jim Beatty-Ohio State 6 Resistors in Series R 1 R 2 + E I Resistors in series carry the same current .

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